Preheat your oven to 175°C (350°F). Line a 9x13 inch baking pan with parchment paper, allowing some overhang for easy removal.
Gently melt the chopped dark chocolate and butter together using a double boiler or microwave in short bursts, stirring until smooth and glossy. Let the mixture cool slightly to avoid cooking the eggs.
In a large bowl, whisk the eggs and brown sugar until well combined and slightly frothy, about 2 minutes. This helps incorporate air for a tender crumb.
Pour the slightly cooled chocolate mixture into the egg mixture, stirring until smooth. Add vanilla extract if using for extra aroma.
Sift in the flour, baking powder, and a pinch of salt. Gently fold the dry ingredients into the wet batter with a spatula until just combined—be careful not to overmix, as this keeps the bars tender.
Pour the batter into the prepared pan, spreading it evenly with a spatula. Dollop spoonfuls of caramel sauce across the surface, then use a skewer or knife to swirl it through the batter, creating ribbons of gooey caramel.
Bake for 25-30 minutes, until the top looks shiny and crackly, and the edges are slightly pulled away from the pan. The center may still be slightly jiggly but will set as it cools.
Remove the pan from the oven and let the bars cool in the pan for about 15 minutes. Then, lift them out using the parchment overhang and transfer to a wire rack to cool completely for another 30 minutes.
Once cooled, use a sharp knife to slice the bars into squares. Wipe the blade between cuts for clean edges. Serve and enjoy the gooey, salty-sweet indulgence.
